The gory business of autopsy has, in recent decades, inspired much popular fantasy, from Patricia Cornwell's Scarpetta franchise to the anaesthetised glamour of Silent Witness. But truth is grislier than fiction, especially if you travel back to the early 19th century, to the grand guignol realities of London six years before Queen Victoria came to the throne. Against a backdrop of Tudor slums, cesspits and public hangings, Sarah Wise's The Italian Boy reopens the grotesque underworld of "resurrectionists" - professional grave-robbers.

Staying in the ground, in one piece, was quite a challenge for the dead, since surgeons needed a constant supply of cadavers for students to dissect, and there just weren't enough criminals being executed. A freshly exhumed corpse could fetch as much as 20 guineas, considerably more than a factory worker's yearly wage. "Offcuts" such as a well-muscled limb, a woman's scalp with the hair still attached or "smalls" (babies) were also lucrative.

Wise's book examines a case that captured the imagination of 1830s London as surely as it recaptures ours - the murder of a street urchin by a gang of body-snatchers who had decided that digging up graves was too strenuous and that hanging around wakes, funerals and deathbeds was too bothersome. So, in semi-rural Bethnal Green, they started what might be called a cottage industry, luring the homeless off the streets of the metropolis and converting them into anatomical specimens. The luckless teenager of the book's title was their final catch; they were arrested when they tried to sell his still-bleeding body to King's College, and their fate was sealed by the boy's autopsy.

A sensational trial followed, which Wise reconstitutes, Frankenstein-like, from sewn-together Old Bailey transcripts, newspaper reports and other contemporary sources. She has a sharp eye and a cool head for the myriad ways in which the court, the surgeons, the police and the pre-Victorian media were bamboozled by their own smug incompetence. But her primary aim is to provide a window into the lives of the hopelessly poor. The bodysnatching case of Burke and Hare in Edinburgh was a tidier, more bourgeois drama, suitable for theatre adaptation, whereas this long-forgotten story is almost ungraspably mucky, full of the casual brutality, alcoholic logic and harsh poignancy of underclass existence.

Wise's bodysnatchers showed an entrepreneurial spirit of which Norman Tebbit would be proud. Once they had their "Thing", they would cart it back and forth across London in search of the highest bidder, haggling with hospital porters, taxi drivers and dentists (to whom a nice set of ripped-out teeth could be sold separately). Ghoulish opportunism wasn't restricted to criminals: we goggle at the chutzpah of the lollipop peddler who, after the murderers' execution, sells candy figurines of the men dangling from a miniature sugar gallows. A seemingly inexhaustible supply of such surreal details makes The Italian Boy compelling reading.

The core mystery, concerning the murderers and their victims, has all the melodrama, plot twists and pathos of Victorian fiction. Wise could easily have crafted a novel out of it, and may have been tempted to do so. Instead, she contents herself with some fictional devices in the racier parts, blurring the distinction between verifiable data and fanciful reconstruction. For example, after a surgeon gives his verdict on the boy's cause of death, "two young trainees, by now feeling faint with tiredness and nausea, stayed in the cold, stuffy room to sew up the corpse". A punctilious reader might reject such nuances as sheer surmise, but there is method, even wisdom, in Wise's embellishments. The sewing up would almost certainly have been left to trainees who, given the protracted duration of the autopsy, would most likely have been tired; the room must surely have been cold and stuffy. Yet even if Wise were mistaken in each of these assumptions, the historically significant matter - the autopsy's findings - is left alone.

This is Wise's modus operandi throughout; she elaborates the backdrop, occasionally allowing anonymous bit-players an unobtrusive role, but doesn't tamper with the evidence or presume to know more about the protagonists than her sources record. Unlike many modern historians, she abstains from unsubstantiated allegations about the inner drives, sex lives and formative childhood experiences of her subjects. In this book, the witnesses speak for themselves, or are allowed to remain silent.

Wise's scrupulous scholarship is also evident in the way she digresses from the central narrative to examine urbanisation, parliamentary reform, vagrancy laws, the evolution of the modern police force and so on. To readers in a hurry to pursue the murder tale, such material may be unwelcome, although one could argue that the alternation of story and excursus conforms to the classic thriller pattern of tension/lull/ tension/lull. Even so, the chapter on the Smithfield meat markets, barely relevant and over-long, suggests that Wise may have succumbed to the temptation to share every juicy detail her research uncovered.

Indeed, the hefty size of this book calls into question her introductory claim about "vanished" history. She declares that "the very poor are persons unknown" whose lives have gone largely unrecorded, then reproduces a wealth of documentary material preserved in libraries and archives. Hardly a "deafening silence that roars down the centuries", unless this overwrought phrase is intended merely to suggest that too few people have been listening. In academia the pressure to justify one's researches with a big contention is great, and Wise's slightly pumped-up preface seems pitched at an imaginary thesis supervisor. She also expounds, in a footnote, a flimsy argument that an anonymous bit of journalism might possibly have been written by Dickens. Such dubious bonuses are redundant in a story as rich and enthralling as this.

Indeed, so enthralling is the case of the east London bodysnatchers that it's hard to believe The Italian Boy is the first book about it since the catchpenny pamphlets that were hawked around the streets in 1831. Wise's debut, written with flair and plentifully illustrated, is history at its most shamefully entertaining.

Michel Faber's The Crimson Petal and the White is published by Canongate.

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